1. Field of the Invention
The present invention relates to a common mode feedback amplifier circuit and method, and more particularly to a common mode feedback amplifier circuit for reduction of signal distortion.
2. Description of the Prior Art
For the requirements of the typical common mode feedback (CMFB) amplifiers, the direct current (DC) voltage level of the output signal is required to be maintained at the same level as the reference voltage, and is to be prevented from being influenced by the DC voltage level of the differential input signals.
Normally, a detecting circuit is included in the CMFB amplifier, in order to detect and correct the DC voltage level of the differential output signals of the CMFB amplifier. When the DC voltage level of the two output signals is different from the reference voltage, the detecting circuit will suitably correct the DC voltage level of the output signals of the CMFB amplifier, until the DC voltage level of the output signals is adjusted to the reference voltage.
However, every detecting circuit of the CMFB amplifier may have its own working limit. Accordingly, once the DC voltage level of the output signals is far deviated from the reference voltage, the detecting circuit may not be able to suitably correct or adjust the DC voltage level of the output signals of the CMFB differential amplifier back to the reference voltage.
When the CMFB amplifiers are working or operating for large signals, deviation of the DC voltage of the differential outputs becomes worse. As such the DC voltage may drift to a higher value or a lower value. In either case the output signals may be distorted accordingly, such that the operating range of the output signals will be limited, and such that the operating range of the input signals will also be limited.
In order to prevent the distortion of the output voltage signals, U.S. Pat. No. 5,729,178 to Park et al. provides a fully differential cascode operational amplifier (OP AMP) having an adaptive biasing circuit to eliminate the effect of slew rate. The amplifier has two outputs V1 and V2 compared with a reference voltage VCMREF. However, when V1=V2 and when V1 and V2 are much greater than the reference voltage, the signals may not be suitably adjusted back to the reference voltage.
The working range may have been improved by Park et al., however, the DC voltage level of the output signals may still be greatly deviated from the reference voltage under certain circumstances.
U.S. Pat. No. 6,448,848 to Altmann discloses a differential transconductor-capacitor (gm-C) circuit employing two transistors to calibrate or adjust the output signals. The control signals are coming from the voltages of a detecting circuit. However, in the detecting circuit, the common mode reference voltage is not compared directly with the input signals, instead it is compared with the intermediate signals, such that the output signals “out and outB” may not be accurately controlled.
When the two output signals “out and outB” may not be completely symmetric to each other, due to the manufacturing procedures of the semiconductor process, the voltage difference may not be suitably improved by the detecting circuit. The detecting circuit may only be used to improve the average voltage of the two output signals, such that the two output signals “out and outB” may have different DC voltage levels, and such that the signal distortions may occur for the large signals.
The present invention is to mitigate and/or obviate the afore-described disadvantages of the conventional common mode feedback amplifier.